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Molecular Genetics > lecture 3 > Flashcards

lecture 3 Flashcards

1
Q

Chromosomal Abnormalities

A
  • alterations in quality or quantity of genetic material
  • 50% of spontaneous miscarriages
  • due to errors during cell division or mutagenic and ionizing radiation
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2
Q

polymorphic phenotype syndromes

A
  • due to insertions, deletions of genetic errors
  • doesnt follow mendels law
  • non-curable
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3
Q

Causes of structural autosomal abnormalities

A
  • deletions
  • inversions
  • translocations
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4
Q

causes numeric autosomal abnormalities

A
  • aneuploidies
  • polyploidiesc
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5
Q

causes numerical sex abnormalities

A
  • aneuploidies
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6
Q

Structural autosomal abnormalities

A
  • less frequent than numerical
  • due to breakage in one or more chromosomes -> incorrect rejoining of segment or loss/gain of material
    -> chromosomal rearrangement
  • main cause: ionizing radiation and other mutagens
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7
Q

balanced and unbalanced structural autosomal abnormalities

A
  • balanced : complete chromosome set, but rearranged
  • unbalanced: additional or missing genetic information
    (includes deletions, insertions, duplications of segments)
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8
Q

Gaps and breakages

A

= areas of DNA thinning
- occur during interphase

  • when breakage does not affect the centromere, a smaller chromosome is formed with centromere and acentric breakage
    -> smaller chromosome has deletion in chromosome part
    -> acentric breakage will disappear in next cell division -> cells with chromosomal deletions
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9
Q

Deletions in structural autosomal abnormalities

A
  • chromosome breaks at one point -> end part of chromosome is lost
  • breaks at 2 points -> middle part is lost
  • deletion can be small or large
  • lost during replication, as the kinetochores attach to centromeres
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10
Q

Duplications in structural autosomal abnormalities

A
  • part of chromosome is duplicated several times
    -> unequal cross-over between homologs or non-homologs
    -> multiple copies of some genes
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11
Q

inversions in structural autosomal abnormalities

A
  • breakage at 2 different parts -> broken segment is rejoined inverted
  • no problems in mitosis
  • paracentric inversion: centromere not affected
    -> no change in chromosomal morphology -> inversion can only be seen with banding techniques
  • pericentric inversion: centromere part of inverted segment
    -> centromere changes position, possible change in morphology
    (submetacentric can become acrocentric)
    -> crossing over in inverted segments results in gametes with abnormal chromosomes
  • final chromosomes have deletions, duplications that are inviable
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12
Q

Translocation in structural autosomal abnormalities

A
  • formation of a new translocated chromosome (derivative chromosome)
  • the chromosomes may be homologs or non-homologs
  • reciprocal (balanced): no loss or excess of genetic material
  • non reciprocal (unbalanced): loss or excess of genetic material
  • individuals with reciprocal translocations are phenotypically normal, but gametes may have unbalanced translocated chromosomes
  • unbalanced translocation occurs when the offspring gets 1 derivative chromosome from a parent, who carries balanced translocation
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13
Q

robertsonian translocation in structural autosomal abnormalities

A
  • when p arms of 2 acrocentric chromosomes break, resulting in q arms with sticky ends
    -> joining of the 2 q arms results in 1 large chromosome with 2 q arms
  • number of chromosome decreases by one
    -> 2 initial acrocentric chromosomes become 1 meta- or submetacentric chromosome
  • p arms usually carry redundant genetic material -> loss does not cause clinical symptoms and carriers of rob translocation are phenotypically normal
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14
Q

Ring chromosomes

A

telomere of each chromosome arm has been deleted -Y broken arms have reunited in ring formation
- breaks are lost -> deleted genetic material
- can result in abnormal gametes
- can still cause syndromes

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15
Q

isochromosomes

A

1 arm is duplicated -> 2 arms of equal length with same loci
the other arm is deleted
-> chromosome with 2 equal identical arms (either 2 q or p arms)

  • excess copies of some genes, loss of some others
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16
Q

dicentric chromosomes

A
  • chromosome with 2 centromeres
  • 2 chromosomal breaks (each carries centromere) that are fused together
  • unstable -> lost during cell division -> loss of genetic material
17
Q

47,XY,+13

A

male with extra chromosome 13 (trisomy 13)

18
Q

46,XX,5p-
or
46,XX,del(5p)

A

female with 46 chromosomes, deletion in p-arm of chromosome 5

19
Q

46,XX,t(2;8)(q21;p13)

A

female with 46 chromosomes
reciprocal translocation between chromosomes 2 and 8 with breakages in segments q21 and p13

20
Q

46,XX,inv(10)(p11q13)

A

female with 46 chromosomes
inversion in chromosome 10 between p11 and q13 (pericentric)

-> pericentric because its between a p and q arm

21
Q

45,XY,-14,-21,t(14q21q)
or
45,XY,der(14;21)

A

male with deletions in chromosome 14 and 21
carrier of reciprocal translocation between q arm of Chrom 14 and 21 (derivative chromosome)

22
Q

46,XX/47,XX,+8

A

female with mixed cells (mosaicism)
normal cells and cells with extra chromosome 8 (trisomy 8)

23
Q

Cri du chat syndrome

A

structural autosomal abnormality
- 1/35,000
- deletions in p arm of Chromosome 5 5p-
- can be terminal or interstitial
- can be inherited due to unbalanced translocation

  • terminal deletion: loss of end of chromosome
  • interstitial deletion: results after two breaks in the middle of the chromosome are induced
  • abnormal tongue, laryngeal growth, mental retardation, elongated face, facial dysmorphism, strabismus, gastrointestinal and cardiac complications, etc
  • size of deletion varies among individuals -> the larger the deletion, the more severe the intellectual disability and developmental delay
24
Q

Beckwith-Wiedemann Syndrome (BWS)

A

structural autosomal
- 1/13,700
- interstitial deletion of chromosome 11 11p-

  • overgrowth, macroglossy, hypoglycemia, umbilical hernia, cancer predisposition, anterior lobe plexus, vascular malformations, organomegaly, etc.
25
Q

Chromic myelogenous leukaemia (CML)

A

structural autosomal
. hematologic malignancy: white blood cells dont mature, proliferate in large numbers
- 3rd most frequent type of leukaemia in adults
- reciprocal translocation between chromosome 9 and 22 t(9q22q)
-> philadelphia chromosome - derivative chromosome

  • fusion of 2 genes : bcr and abl1
    -> chimeric protein BCR/ABL1 : over-activity -> oncoprotein, which induces proliferation, differentiation and survival
26
Q

Numerical autosomal abnormalities

A
  • most frequent mutations in humans
  • due to failure of homologs to separate during anaphase in cell division
  • mostly in meiosis, but can occur in mitosis as well
  • phenotype varies (nr and type of chromosome)
  • abnormal nr of chromosomes in a cell increases the risk of having more abnormalities in the forthcoming cell divisions
  • only 3 disorders are viable: trisomy 21, trisomy 13 and trisomy 18
  • various mechanisms:
    – non-disjunction
    – delay during anaphase
    – polyploidization
27
Q

Non-disjunction (numerical autosomal)

A
  • most important mechanism
  • chromosomes remain together during anaphase -> move toward the same pole
  • mosaic cells: normal cells (2n), trisomic cells (2n+1) and monosomic cells (2n-1)
    -> aneuploidy: excess or lack of small number of chromosomes
  • meiosis 1:
    homologs dont separate -> 2 gametes with n+1 (trisomic zygotes) and 2 gametes with n-1 (monosomic zygotes)
  • meiosis 2:
    sister chromatids dont separate -> 1 gamete with n+1, 1 gamete with n-1, 2 gametes with n (normal)
28
Q

delay during anaphase (numerical autosomal)

A
  • chromosome moves toward pole with delay
    -> aneuploidies
29
Q

polyploidization (numerical autosomal)

A
  • homologs appear in cells more than 2 times -> presence of multiple series of homologous chromosomes
  • polyploidy: any multiple of basic haploid chromosome number other than the diploid number -> more than 2 sets of chromosomes
  • only triploidy (3n) has been observed in humans
  • incompatible with survival or normal human development
30
Q

Trisomy 21 (downs)

A
  • 1/800
  • life expectancy over 30 years
  • round face, upward slant eyes, epicanthal folds, brushfield spots on iris, enlarged tongue, minor ear anomalies
  • mental and developmental retardation, hypotonic muscles, short hands and fingers, congenital heart disease, alzheimers, leukemia risk, premature aging
  • females can be fertile, males are infertile
  • correlation to mother’s age (accumulation of mutations over the years in ova)
  • 95% is trisomy 21
    due to meiotic non-disjunction (95% during meiosis I in ova, 5% in meiosis I in sperm)
  • non-inheritable
  • 47,XX,+21)
  • 4% due to robertsonian translocation
  • 1% mosaic down syndrome
  • partial trisomy
31
Q

Robertsonian translocation in trisomy 21

A
  • 4% of trisomy 21 cases
  • individuals with 46 chromosomes, one chromosome from rob translocation between q arm of 21 and q arm of a acrocentric chromosome (usually 14 or 22)
    -> derivative chromosome replaces on of the normal acrocentric chromosome -> patient is trisomic for 21q

** 46,XX or 46,XY,-14,t(14q21q)**

  • no correlation with mothers age
  • increased risk when mother (or father) is carrier of rob translocation t(14q21q)
  • carrier has 45 chrom -> chrom 14 and 21 are lost, but replaced by the derivative chromosome 45,XX or 45,XY,-14,-21,t(14q21q) but phenotypically normal
  • 6 types of gametes formed by carrier:
    3 are not viable -> unbalanced
    3 are viable -> normal, balanced (gives rise to a carrier) or unbalanced (gives rise to trisomy 21q, when it has the chrom with translocation and one chrom 21)
32
Q

mosaic down syndrome

A
  • 1% of the cases
  • patients come from zygotes that have trisomy 21
  • milder phenotype
  • high diversity in the phenotypes due to variable proportion of trisomy cells in the embryo during first stages of development -> more trisomic cells, more severe phenotypes

a) error in mitotic division of normal blastocyst (non-disjunction) -> trisomic cells
b) error in mitotic division of trisomic blastocyst (loss of chromosome) results in normal cells

46,XX/47,XX,+21 or male

33
Q

partial trisomy in down syndrome

A
  • rare
  • formed by trisomy in segment of q arm in chromosome 21
  • region found in chromosome banding area q22 and contains 50-100 genes (incl. enzymes for purines, superoxide dismutase ans alpha-crystallin)
34
Q

Edward’s syndrome

A

47,XX,+18 or 47,XY,+18
- trisomy 18
- 1/5,000
- errors during gametogenesis

  • 5% mosaic trisomy 18: milder symptoms
  • rare partial trisomy18: translocation of q arm
  • more frequent during conception but 95% result in spontaneous abortions
  • embryos dont survive more than few months, rare cases have survived 15 years
  • mental and developmental retardation, heart problems, abnormal kidney function, short sternum, micrognathia, dysplasia in ears
35
Q

Patau syndrome

A

47,XX,+13 or 47,XY,+13
- 1/16,000
- errors during gametogenesis

  • 20% partial trisomy 13: translocation of segment (sometimes inherited)
  • rare mosaic trisomy 13: milder symptoms
  • embryos dont survive more than 6 months
  • mental retardation, abnormal development, heart problems, facial dysplasia, urogenital system problems, cleft lip and palate, polydactylyl
36
Q

Abnormalities Sex chromosomes

A
  • most common in humans
  • either structure or number
  • in all sex cells or mosaic form
  • most common trisomies: XXY, XXX, XYY
  • most common monosomy: X (turner syndrome)
37
Q

Mosaicism

A
  • due to abnormal mitotic division during initial stages of embryogenesis
  • different number of chromosomes in different cells -> genetically different cell lines, that arise from a single zygote
  • usually monosomies or trisomies
  • autosomal or sex chromosomes

Molecular Genetics (5 decks)

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